1. Field of the Invention
The invention relates to signal processing and in particular, though not exclusively, to the correction of drift and wow and flutter in signals that have been recorded.
2. Discussion of Prior Art
When a signal is transmitted through a medium, there may be fluctuating and drift time delays imposed on the signal due to changes in the propagating medium. Such a medium may have an analogue in a tape recording system where a signal is subjected to delays during recording and playback.
When making recordings of signals it is impossible to maintain a perfectly constant tape speed because of limitations imposed by mechanical and environmental conditions. There will therefore be short term variations (wow and flutter) and also long term variations (drift) in the recorded, and subsequently, played back signals. The short term variations appear as a recognisable frequency modulation on the recorded signals. The maximum deviation of this change in frequency will depend upon the mechanical precision of the recording system as will the frequency of modulation (rate of variation). Variation frequencies up to 10 Hz are normally referred to as `wow` whilst those above 10 Hz are called `flutter`. Very slow variations and steady state errors (long term average speed errors) are called `drift`. These speed induced drift errors are responsible for creating timing inaccuracies during playback and from recording to recording etc. Drift is likewise caused by the limited precision of the mechanical drive but it is also dependent upon the elasticity properties of the tape, tape tensions, the surface properties of the tape, the ambient temperature, air humidity and upon any variations in mains supply.
Wow, flutter and drift performances can often be held within acceptable limits without resorting to sophisticated systems since the human ear is unable to detect some short term frequency variations (wow and flutter) with the same level of perception over the entire audio frequency range. Thus it is possible to minimise the apparent wow and flutter by ensuring that the predominant frequency of variation is not close to 4 Hz, the frequency of maximum perception.
When used for recording and playback of data, however, modern electronic systems can be susceptible to very small frequency, phase and time discrepancies. Thus, when recordings are made of particular `time synchronous` and phase modulated signals, these signals will be corrupted by the relatively very much larger phase distortions produced by the wow and flutter, and the drift will cause time slip and frequency shifting in the playback signal.